1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image is formed by an electrostatic electrophotographic system, and more particularly relates to an image forming apparatus whose patch image forming portion for toner density control is improved.
2. Description of the Prior Art
FIG. 3 shows a schematic drawing of a reproducing apparatus. Reference numeral 1 denotes a case member. A document glass plate 2 and a cover 3 for covering the document glass plate 2 are fitted to the upper surface of the case member 1; A rod-like exposure light source 4 for effecting an exposure scan in a direction indicated by an arrow a, mirrors 6 to 9 for guiding light issued from the light source 4 and reflected from a document 36 placed on the document glass plate 2 to a photosensitive drum 5 and an image forming lens 10 are disposed on the lower surface of the document glass plate 2. Disposed around the photosensitive drum 5 are a charging electrode 11 for charging the photosensitive surface of the photosensitive drum 5 to a high voltage, a charge eliminating lamp 12 for eliminating unnecessary charges among the charges which are given by the charging electrode 11, a developing device 13 for developing a latent image formed by the exposure with toner, a transfer electrode 14 for transferring the resulting toner image to transfer paper, a separating electrode 15 for separating this transfer paper from the photosensitive drum 5, a photoelectric sensor 16 of a reflection type for detecting transfer paper which is wound on the photosensitive drum 5 and for detecting the density of a patch image, a cleaner 17 for removing the toner remaining on the photosensitive surface of the photosensitive drum 5, and so forth. Reference numeral 18 denotes a conveyor belt for conveying transfer paper after separation; 19 is a fixing unit for thermally fixing the toner image on transfer paper; 20 is a paper discharge tray; 21 to 23 are paper feed cassettes in which stacks of transfer paper having different sizes are loaded, respectively; 24 to 26 are feed rollers for feeding one sheet of transfer paper, respectively; 27 to 29 are first feed rollers for feeding transfer paper fed by the feed rollers 24 to 26 to guides 30 to 32, respectively; 33 are second feed rollers for establishing a timing between the edge of the fed transfer paper and the edge of an image formation region on the photosensitive drum 5; and 34 is a guide for sending transfer paper to the photosensitive drum 5. Reference numeral 35 denotes a density reference plate of black color for controlling the toner density; 36 denotes a document placed on the document glass plate 2; 37 denotes a brake sensor; and 38 denotes a holding frame for holding the light source 4 and the mirrors 6.about.8.
In this reproducing apparatus, after the document 36 is set onto the document glass plate 2, the document glass plate cover 3 is closed and then a copy button on the operation panel (not shown) is pressed down. Thus, the light source 4 moves at first in the backward direction (the return direction) opposite to that indicated by the arrow a and the size of the document 36 is detected. (A detailed explanation of the size detection is omitted because it has no direct relation to the present invention.) When the movement of the light source in the backward direction is continued, the holding frame 38 of the light source 4 is detected by the brake sensor 37, so that the holding frame 38 of the light source 4 receives a braking force and is decelerated, reversed at a reversing point P1 shown in FIG. 4 and then sent traveling in the forward direction indicated by the arrow a.
In the movement of the holding frame 38 in the forward direction, the light source 4 continues to illuminate the density reference plate 35 and to illuminate the document 36. Accordingly, a patch image (toner image) 5b is formed in a non-image forming region 5a of the photosensitive drum 5 as shown in FIG. 5 and a document image (toner image) is formed in the subsequent image forming region 5c.
If the magnification of reproduction is low, the exposure scan speed of the light source 4 is high if the rotational speed of the photosensitive drum 5 is constant, and consequently the time for exposure of the density reference plate 35 becomes short. In other words, the exposure finish timing of the density reference plate 35 become earlier, thereby shortening the length l of the patch image 5b in the drum rotating direction.
Accordingly, this causes a problem in that the number of sampling points (a plurality of points in the drum rotating direction to be sampled) for detecting the density of the patch image by the photoelectric sensor 16 becomes smaller. Control over the toner density and the reliability of data of the patch image obtained drops.
This problem can be solved by increasing the length of the density reference plate 35 in the exposure scanning direction, but there is an inevitable limit to the increase in the length of the density reference plate 35 because the available space is extremely limited in order to satisfy the requirement of reducing the size of the reproducing apparatus.
A reproducing apparatus of the type wherein the exposure is started after resting at the home position, in the back start system, has been suggested to overcome this problem. If this system is adopted, however, the number of copy sheets per unit time (CPM) can not be increased.